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Tuesday, 11 August 2009

Tar - what is it?

Up until the 1820s - and often afterwards - 'tar' meant a distillate of wood. Coal tar was 'mineral' tar. Words and what they mean can change a lot.

Tar distillers - working with both wood and coal tars - used a wide variety of words to describe the tar fractions, which they were dealing with. There is a great deal of confusion because the words they used have changed in meaning over the last two centuries. Before discussing the gas companies, the tar distillers and their work it might be useful to look at the language which they were using.

What was tar? While that may appear obvious to us, in the early nineteenth century it was less clear what the word meant. 'Tar' then was the name of a black sticky substance derived from wood and which was widely used, particularly in shipping. When coal gas was first made a solid substance was recovered when the new gas was washed. This was like 'tar' and seemed suitable for use in the same way and so it began to have the same name.

This ‘coal’ or ‘mineral’ tar became available at the end of the nineteenth century when ideas about such substances were changing. Increased research into the use of coal tar was undertaken because of its sudden availability, This led on to new ideas about what exactly ‘tar’ was. Two strands of research - for practical use and for ideas - ran in parallel and fed off each other.

When scientists identify new substances they coin names for them. One generation's batch of names is superseded by the next's. Such names often change their meanings in the process. A good example of this is 'creosote' - a word used in the 1990's for a coal-tar derivative commonly used as a preservative paint, which is available in every DIY store. The Berlin chemist, Reichenbach, coined the word 'Kreosote' in 1830, to describe a product of wood tar and it was not used to describe a coal tar distillate until after 1836.

Frederick Christian Accum, whose role as an advisor to the Chartered Company has been mentioned above, described tar manufacture in two books published before 1820. As the Chartered’s consultant on chemistry what he says can be taken as a good guide for the thinking of the company at that time. Tar, he said, was made as part of the process of gas manufacture, during which it was 'deposited in a vessel to receive it'

He went on to describe some of things which could he made from the deposited tar. It should be condensed to get 'essential oil or oil of tar'. Through evaporation 'coal oil' could be obtained - he described this as a 'yellowish inferior kind of naphtha'. Otherwise, he said, it could be boiled for 'pitch' which by the 'evaporation by heat gives asphaltum'.4 What did it all mean?

Georg Lunge writing on tars a century after Accum discussed this problem of the names by which substances were known. As an illustration he provided a definition of 'pitch'.

'This denoted many substances which have nothing to do with coal tar.. such as natural bitumen'.

He went on to describe 'asphalt' as

'the residue in the retort once the light oil is distilled off.. but to tar distillers it is 'ordinary pitch' mixed with other things.. and in the past it meant 'bitumen'.

All of these words for tar distillates have meanings that were changing throughout the period of the early gas industry. Obviously, this leads to some difficulty in understanding the activities of those who used them. It is not practical to give a complete analysis of every term. Here are some comments on two of them.

'Pitch' is perhaps the simplest. It was understood to mean the residue left after distillation of the first 'coal oil'. It is the substance, which the British Tar Company sold, and which, as we shall see, the Chartered Company tried to sell to the navy.

Many complications arise in the definition of 'naphtha', a word in common use in the nineteenth century. In a modern dictionary it is defined as a 'vague name for the inflammable liquid distillates from coal-tar'.Earlier use was more precise ‑ a nineteenth century chemical dictionary defines it as 'the volatiles observed in substances'.The 'naphtha' from tar is said to have been identified in 1779 by a Giovanni Fabricini although 'until 1823 it was called coal-oil, volatile-oil and spirits of tar'.

Gas works engineers used the word 'naphtha' for some coal oils in the 1820s, while labourers in the works referred to it as 'fat'. Meanwhile 'chemists like W.T.Brande' insisted that “'naphtha' only applied to the products of distillation of natural bitumen'. Thus 'naphtha' meant one thing to scientists and another to those working in the field. In the twentieth century the precise meaning, already obsolescent in 1820, has been lost.

One of the clearest definitions of what tar distillers sold is the earliest. In 1789 the British Tar Company produced a sales booklet which described the different tar fractions sold. It described: 'Tar No 1... Raw tar from the kiln. No.2. Freed by the kiln from added water.' This was followed by three more 'marks'. This easily understood definition of types of tar by a simple gradation is very rare - usually there is an ever-changing mass of names, most with several meanings.

Winsor and his original supporters advertised their intentions for by-products before the Chartered Gas Company began business. When they described tar it is far from clear whether they were advertising the use of raw or processed tar. They generally failed to mention the need for processing at all. They tried to explain what gas works tar was ‑or rather what it was like, or what they thought it was like. To do this they said that coal tar 'resembles common tar' and was 'applicable to the same uses as tar from Sweden - that is tar made from wood.

The British Tar Company sold 'if ordered' tar which was completely unprocessed. Tar of this sort was known to be a preservative if it was painted on wood ‑ Phillipe Lebon, the subsequently murdered French scientist and tar manufacturer, had recommended it as such in 1799.15

A Benjamin Cook, of Birmingham, wrote to the press in 1808 to describe his experiments on the use of coal tar. Cook appears to have been a toy maker or a brass founder with an interest in gas manufacture - in December of the same year he also published an account of his gas making apparatus. He thought raw coal tar would make 'an excellent coating for all out of door work such as gates, fencing and paling'. Other letters and articles on this theme followed. In 1809 an article in the Edinburgh Review pointed out that tar was 'useful as a coating to prevent damage by wind and rain to timber'.

Ten years later such use of raw tar for outdoor work was again put forward as a good idea when Mechanics Magazine published a number of letters extolling its use by farmers. In 1827 a Christopher Davy described the uses of coal tar ‑'a most valuable product'. It could be used to paint on gates and doors; and for farmers used on hurdles and as a roofing material. Another letter came from 'Amateur Mechanic' who had used tar to paint his roof tiles and as a result they no longer blew off or let in water.

Why were such letters written? They can only have been puffs ‑ were Christopher Davy and 'Amateur Mechanic' trying to sell something? As the century progressed such uses of coal tar must have become increasingly better known and their services were no longer needed. Their letters provide a pointer to the sort of ways in which coal tar, in the days of the early gas industry would have been used.

Before 1800 varnish was made from various mixes of oils, many secret, and used by the 'carriage trade'. Coal tar varnishes were rather rougher than that. In the early days of the gas industry a number of mixes for painting on wood were known as 'varnish' or 'tar varnish' or 'naphtha varnish'. In the early days of the gas industry the British Tar Company's grades of tar sold before 1800 had included a 'pitch varnish' and 'an oil used for varnish'. In the 1820s Frederick Albert Winsor, Jnr. described 'essential oils in different qualities, applicable, instead of turps, in painting from the finest to the coarsest' and ‘asphaltum for...... japanning and varnish of the highest gloss'.21 There were, no doubt, many recipes.

Initially coal tar varnish seems to have been considered as a substitute for other, more expensive, ingredients. In 1809, Benjamin Cook had said that it could substitute for 'the tar spirit brought from Russia and of vast importance to manufacturers'. He described the first distillation of the tar as 'a substitute for turpentine' because it 'takes as beautiful a polish'. In addition, he said, the pitch left after the second distillation ' forms an ingredient for black varnish'. Cook had produced a 'waiter.. japanned with varnish made from the residuum and the volatile oil’ and said that he hoped to put items like this into production.

The subject of varnish had been investigated before the early gas industry made tar for varnish more widely available. Dundonald used tar to make 'varnish' in 1789 and Sir John Black, who examined Dundonald's work, described how 'volatile oil was distilled from boiled tar for it.

In 1808 Cook gave details about how varnish was made. He produced three specimens of spirit, each the product of a re-distillation and explained how these distillates could be used instead of the more familiar oil of turpentine. It made a varnish, he said, in which 'there appears to be no manner of difference'.

One 'manner of difference' however, may have been the smell and the 1809 Parliamentary Enquiry panel questioned Accum about this. Of course, he denied any bad smell, but the question is an indication of the image which coal tar varnishes always seem to have had as a cheap, and perhaps nasty, substitute for more expensive traditional oils like linseed and turpentine.

This was a bad start. Coal tar took a long time to lose an image of something rather nasty the use of which often needed to be concealed. The next chapters will look at how the Chartered Gas Company and its competitors went about marketing the tar, which, they had been assured, would bring them in a vast profit.